1. Field of the Invention
This invention involves the fabrication of conductor-clad composites using molding compounds and associated molding techniques.
2. Description of the Prior Art
Despite the advent of integrated circuitry, the much larger scale printed wiring board technology is still in very substantial use in those areas where miniaturization is not critical. A prevalent technique for fabricating copper-clad composites, especially those composites which are used as printed wiring board, involves the manufacture of resin-impregnated woven reinforcement. Subsequent to impregnation of the woven reinforcement with an appropriate resinous compound, the material is passed through a heated oven where the polymer in the resinous compound is caused to partially crosslink thereby yielding a somewhat tacky structure commonly referred to as a "prepreg". The prepreg is cut to appropriate size and a number of such sheets are pressed in a suitable open platen laminating press at pressures of from 15 to 300 pounds per square inch (psi) and at temperatures of from 250-550 degrees Fahrenheit for approximately an hour. The laminating process may involve simultaneously the application of an appropriate conductive foil sheet (e.g., copper) to one or both sides of the substrate, upon which a printed circuit may be subsequently defined. Any vapors that form during the laminating process may escape through the side openings in the open platen press, and consequently, have little deleterious effect upon the bonding of the copper and its resultant smoothness. This laminating technique requires a significant amount of time (approximately one hour) due to the lengthy layup and curing steps. Material costs, primarily associated with the woven glass reinforcement, and the lengthy processing time make the cost of the product prohibitive for many applications.
Suggestions for lowering the cost of copper-clad composites include the use of bulk molding compound to form an appropriate substrate to which a conductive cladding is subsequently bonded (see, for example, Insulation Circuits, November 1976, page P-41). Such a process, however, retains the costly dual-step nature of the prior process (i.e., substrate fabrication and subsequent bonding). In addition, use of the resultant conductor-clad composite as printed wiring board has been largely unsuccessful, in part, for lack of proper physical, mechanical, electrical, and chemical properties.
The pultrusion process has recently been applied to the single-step fabrication of copper-clad composites (see U.S. Pat. No. 4,012,267). The good quality of the copper bond in the pultrusion process is due partly to the "open" nature of the pultrusion press which allows for venting of vapors produced during curing. While the single-step pultrusion process is highly effective, cost reduction is still an object of significant pursuit.